To make it easy, we recommend using Smart Albums (www.smartalbums.com) for your page layout. Couture sizes are built right in, so you can just pick a size and start designing.
Also, Fundy Album Builder (www.fundysoftware.com) has the Couture sizes built in as well.
We have a plug-in built for Aperture, although Aperture is not going to be further developed by Apple. More info is here
If you are planning on using your own design program like Photoshop or InDesign, you may need to set up the page/canvas specifications.
We try to keep the page layout process very simple and easy: You'll just start with a page sized at the exact size of the book you want to order at 300 DPI. For instance, if you are designing an 8x10 book, you'll start with an 8x10 page at 300 DPI.
We do trim about a 1/16th of an inch from the outside edges on the books with the straight cut edges, and an 1/8th on the books with the hand-torn edges. It's not very much, so generally I wouldn't worry about it. The only time I'm concerned with the trim affecting the image is if you have a full bleed image with little to no headroom, or text. In those cases please give us a little extra room.